Drilling holes in slate and granite

What kind of drill bit do I need for a) slate and b) granite? (I have a
small drill press)
How do I proceed - one hole, or several - drilling out gradually to
increase the diameter of the hole? I need to drill out a hole of about
8mm.
Any difficulties I should forsee, like slate splitting?
Do I need water cooling? Depth of slate would be either 10mm or 20mm.
Is this fairly fast or very slow and laborious? How is granite compared
to slate?
Thanks - never drilled into these materials before. Andy

Slate is easy and very forgiving - just use a masonry bit. You won't
need cooling. A pilot hole will make it easier to get the hole exactly
where you want it, but is unnecessary if you can clamp the workpiece. It
should only take you a couple of minutes.
Granite is much, much, much harder. I'd use a masonry SDS (assuming the
same depth and hole size), starting with a 4mm and working up to an 8mm.
But you need to support the granite very well to avoid breaking it.

But you need to support the granite very well to avoid breaking it. >>
In terms of splitting, is granite better or worse than slate? I'm
considering using one or other for a face plate for a diy hi-fi
amplifier. If there's a risk of splitting, I presume I need to use an
aluminium plate behind it for structural reasons, and glue the slate to
the alu (???what glue). Prefer to avoid it - what do you suggest?

Commonwealth war graves in France have slate headstones from the
Wynncilate quarry. Saw pallet loads of them last time I was there 'cos a
bunch of French oiks had smashed up a load of Tommies' resting places.

Slate comes in many grades, roofing slate is very soft and splits
cleaning and easyly (thats how they make 'em). Some slates are very
dense, hard and don't split. We have some 3cm think lumps of Cumbrian
Green Slate for the hearth, tough as old boots.

And building and worktops, though I can't see why people want granite
worktops unless you are a pastry chef.

I have never understood the need for granite or marble pastry surfaces
in a kitchen. After all, they will be at ambient room temperature either
way.
Now if you were to put a marble slab into the fridge before use, I could
understand that for rolling out pastry.
Dave

They are totally different and break in different ways. Slate
delaminates, splitting along the layers. Granite just breaks.

Presumably your 8mm holes are for mounting pots, switches, indicators
etc? Which means you'll need a few of them? I would use slate, without a
doubt. It'll be much less work. No need for a backing plate.

They're chalk and cheese.
Slate will make holes if you stare at it hard. Your problem is to avoid
it splitting. Granite is really hard work and you need just the right
tools. Slate is also highly variable - Lakeland slate is almost like
granite, South Wales slates is flakey junk and a nightmare, North Wales
slate is somewhere in between -- prone to splitting, but reasonably
hard and stable.
For slate I'd suggest non-hammer drilling with an abrasive drill bit
for large holes, or a simple glass drill carbide leaf bit for small
holes. Any drill should cope, even hand drills, but watch out for
wobble causing splitting. Slate should be supported underneath with
soft scrap wood or heavy corrugated cardboard over wood if uneven
For a small number of 8mm holes in softer slate, I'd use a sacrifical
twist drill and expect heavy wear on it. One of the brad-point wood
drill bits with twist flutes but outside spurs as well, ideally TiN
coated, will drill slate well and gives a good edge. Use your drill
press for rigidity (those brad points are hard to start by hand, until
the points sink). You can drill it with normal metalworking twist
drills too, but expect heavy wear.
If you use a masonry drill and non-hammer action, then sharpen it
first. Most masonry drills are sharpened with impact chisel edges, not
cutting edges. For slate it will matter.
Granite can be drilled with an SDS masonry drill and hammer action,
more slowly with a non-SDS hammer drill, or non-hammer with a diamond
core drill for large holes. So you probably can't use your bench drill
for any of these. Diamond drilling of granite may need coolants, extra
slow speed, or high torque (with a safety clutch). Ask the drill bit
maker.
You can also chain drill granite, then break out between holes with a
chisel. This is for 1 1/2" and upwards though, not small holes.
For 8mm holes in granite, then I'd use a standard masonry bit and SDS.
There'll be chipping around the edge though, maybe 15mm affected
radius. If you _must_ have a smooth hole, go with wet diamond. Try to
drill from both sides, as you'll see even more chipping on the exit
hole.
Wet coolants need to be generous, or else they just make a sticky paste
that grabs onto the drill. I probably wouldn't bother, except for
diamond in granite, production in slate, or if your diamond drill maker
recommended it.

Try your local plumber's or tiling or kitchen/bathroom
retailer/merchant.
I had to drill through porcelain and granite tiles on my last job and
the 'spade' shaped bits were useless. I ended buying a diamond crusted
bit which worked beautifully... no stressing or straining... no burning
or 'glowing' of the bit or material.
have a look here...
http://www.toolshopdirect.co.uk/shopscr3098.html
HTH,
deano.

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